An electrically variable hybrid transmission has been proposed for vehicles to improve fuel economy and reduce exhaust emissions. The electrically variable transmission splits mechanical power between an input shaft and an output shaft into a mechanical power path and an electrical power path by means of differential gearing. The mechanical power path may include clutches and additional gears. The electrical power path may employ two electrical power units, or motor/generator assemblies, each of which may operate as a motor or a generator. With an electrical storage system, such as a battery, the electrically variable transmission can be incorporated into a propulsion system for a hybrid electric vehicle.
The hybrid propulsion system uses an electrical power source, such as batteries, as well as an engine power source. The electrical power source is connected with the motor/generator units through an electronic control unit, which distributes the electrical power as required. The electronic control unit also has connections with the engine and vehicle to determine the operating characteristics, or operating demand, so that the motor/generator assemblies are operated properly as either a motor or a generator. When operating as a generator, the motor/generator assembly accepts power from either the vehicle or the engine and stores power in the battery, or provides that power to operate another electrical device or another motor/generator assembly.
Additionally, the stators for each electric motor/generator assembly contained within the electrically variable hybrid transmission may each require differing rates of cooling that are dependent on the duty cycle of each motor/generator. The cooling of the stator is typically performed by bathing the stator with a calibrated flow rate of transmission fluid allowing the heat generated by operation of the motor/generators to be transferred to the fluid. A continuously high cooling rate is simple to implement, however, additional pump loads and spin losses may produce a decrease in efficiency over a selectively variable motor/generator cooling system.